Recently, an RFID (Radio Frequency IDentification) system has been applied for inventory management, merchandise management and distribution management. An exemplary RFID system is configured as follows. A host computer and a reader/writer are connected. A memory having a built-in antenna, called tag, is attached to a managed object. A variety of information related to the managed object (managed object information) is stored in the tag. The managed object information is transferred between the tag and the host computer via the reader/writer. The managed object information in the tag is read out to the host computer, and the managed object information in the host computer is written in the tag. Thus, the managed object information realizes the traceability of the managed object.
Preferably, the antenna employed in the RFD system has a wideband characteristic, a compact size and low profile. It is also preferred that the antenna performance is immune to the property of a member to which the antenna is attached.
There are various proposals for realizing antennas as described above. For example, a proposed antenna has planar antenna elements that are formed on a dielectric substrate and have different resonance frequencies, in which the antenna elements are coupled at a feed point via a transmission line for impedance matching (see Japanese Laid-Open Patent Publication No. 2006-287452). Another proposed antenna functions as a slot antenna in the vicinity of a metal surface and functions as an ordinary antenna away from the metal surface (see U.S. Pat. No. 6,914,562).